General Feeling on Deep Air

[JasonP]

Originally Posted by Garf

Explain to me why, if we've already had a couple of CCR divers on here say they use trimix all the time, some even in the pool, becuase it's so cheap, and they don't need much (please see Mark Chase and Nigel Hewitt's post for reference) then how come you believe I am incorrect to say it's appropriate below 30M on open circuit. Is there something about CCR diving that affects the levels of narcosis, or am I right in saying that using mix on open circuit when narcosis occurs is the only ideal gas, and that anything else is just a compromise due to price and availability.

Well for a start, CCR divers aren't decoing on their backgas.

Plus they're not choosing their ideal gas either. They're picking the one that allows them to not worry about having the wrong mix if their previous dive was cancelled and will be suitable, but not ideal, for everything they think they're likely to do.

Jason

[Mdemon]

"Feeling on deep air"?

Focused. You need your wits about you on air. Same depths on helium require more discipline on deco. So, doable on air if nothing out of the ordinary. Same on helium but deco needs to be very accurate.

Nothing wrong with safe deep air experience. However, helium is the tool of choice for deepest dives. More so on CCR.

[Mark Chase]

Originally Posted by JasonP

Well for a start, CCR divers aren't decoing on their backgas.

Plus they're not choosing their ideal gas either. They're picking the one that allows them to not worry about having the wrong mix if their previous dive was cancelled and will be suitable, but not ideal, for everything they think they're likely to do.

Jason

Sorry Jason, I cant speak for others but in my case I dive mix all the time because it doesn't hurt and its convenient.

Past 30m I dive it on purpose because it improves the quality of the dive

When I dived OC id use an END of 30m for my gas choice. Now on CCR I use 20m. This is totally a cost in money and deco V quality of dive issue. The CCR reduces the deco to a minimum so I can afford to run richer mixes than on OC. The gas usage is so nominal the gas cost is less than an air dive even on 14/60.

If people out there can afford to spend £40-50 on 21/35 and 50% to do a 35m OC dive then good luck to them. I would if I could.

There is all the usual guff about cheep fills and air tops, but this is irrelevant to people like me who don't have access to it and who prefer to come up with virtually empty tanks. Its £40-50 for a 35m mix dive +£45 for the boat + petrol etc. So £100+ for a dive. It's got so expensive these days i can well understand why people are saving money on things like gas.

Point is its saving money balanced against increased risk. There is no argument that the risk of doing a 35m dive on air is greater than the risk on
trimix in all aspects EXCEPT the ascent where the situation is reversed and air becomes the safer product.

There is also no argument that mix is a damed expensive way to dive OC.

ATB

Mark Chase

[Depth-junkie]

deep air has it's time and place, unfortunatly UK 50m+ regualr is not that place.

I went down the deep air route before switching to gas, 50-60m dives. Some were good, others shite.

As Darren has said, some dives are perfectly doable on air I.E Salsette, Moldavia etc. If venturing past 45m then yes mix is the way forward.

Everything past 30m does not have to be on trimix OC unless you have a big cheque book, CCR it really is not an issue.

A few weeks ago we did the bottom of Dotty on 10/50, this gave us an END of 41ish meters. This was totaly acceptable to us and narcs were not an issue, any deeper and i would use a bit more helium. So the question you should be asking is what END is acceptable to you.



ATB
Gareth

[Phoenix]

I knew that I wanted to do a 'mix course but couldn't see the point of paying for Normoxic then Advanced Trimix, the calcs are the same after all. I took the TDI ER course, in warm water admittedly, the Instructor was narced out of his box but the other student and I were ok, most entertaining. I then did some dives to meet the requirements to take the ANDI Full 'Mix course and started diving using Trimix.

I have done one 50m dive in the UK using air dil in an Inspiration, it was ok and I'd do it again if I had to but I far prefer diving CCR on 'mix. I now have banked 'mix and boost that into my dil cyls, this means I dive 'mix all the time and for the small cost of it I am happy to keep doing so.

Ian

[JasonP]

Originally Posted by Mark Chase

Point is its saving money balanced against increased risk. There is no argument that the risk of doing a 35m dive on air is greater than the risk on
trimix in all aspects EXCEPT the ascent where the situation is reversed and air becomes the safer product.

Except that there's years of research and empirical data about decompression theory on air. The same cannot be said of helium. Most of us are using algorithms based on Buhlmann tables he himself labelled experimental.

Jason

[NotDeadYet]

Like Mark says, I stay on the same dil all the way up. It's nothing to do with being able to dive different sites, it's to do with comfort. As for deco considerations, an OC diver doing a switch to a stage will probably have less deco than a CCR diver staying on helium dil all the way up.

Originally Posted by Clare Gledhill

I do find it interesting that GUE divers get quite a bit of stick for buying trimix for the 30-40 metre range yet most happily accept that RB divers can, and do, choose to dive mix on all dives with an END more shallow than GUE recomend as a base line.

I must admit, I used to think "you do what?". Now I can't imagine not diving with some amount of helium. Deep air means build up dives, having a clear head means I can get in the water and go and dive. I will not dive on a rebreather in a situation where I won't have a clear head. The risks are horrendous, especially on a manual RB.

Cheers,

Stuart

[Ivon]

After a brown trousers moment at 50m on air an thinking I was seriously going to die I won't dive past 35m on air now.

Out of interest were do you guys get your J cylinders for premix? I know Lakeside does them but it's a bit of a trek for me. Do you have them cleaned an in test or not bother?

PM me if you like.

[Mark Chase]

Originally Posted by Ivon

After a brown trousers moment at 50m on air an thinking I was seriously going to die I won't dive past 35m on air now.

Out of interest were do you guys get your J cylinders for premix? I know Lakeside does them but it's a bit of a trek for me. Do you have them cleaned an in test or not bother?

PM me if you like.

I have a 300 bar 12 of 18/45 and a 235bar 15 of 18/45. I also have a 12 of 14/60. With a booster pump this lasts for a years diving easy.

I have a J of 02.

ATB

Mark Chase

[Ivon]

Might just fill some 15s then as I have access to a Haskel, Js of O2 an He and compressor through the club, just been lazy as it's at least a 5 minute drive from my house

But it would be nice to just go in the garage and fill from a J.